This invention relates to circuits for filtering electrical signals and, more particularly, to magnetically tuneable circuits for filtering high frequency electromagnetic signals propagating in a waveguide. Specifically, the invention in one embodiment is directed to magnetically tuneable, four-ferrite-sphere waveguide bandpass filters having high off resonance isolation.
Generally, bandpass filters transmit electrical signals within a given frequency range and reject electrical signals having frequencies which lie outside the given frequency range. One known type of bandpass filter is a variable frequency bandpass filter whose frequency passband is altered by controlling the reactance of the circuit parameters of the filter. Such variable frequency bandpass filters are utilized, for example, as preselectors for swept-frequency signal analyzers, such as the HP 8566B or 8562A signal analyzer or the HP 71300A modular measurement system available from Hewlett-Packard Company, Signal Analysis Division, Rohnert Park, California.
One type of variable frequency bandpass filter is the magnetically tuneable filter. In this filter, the frequency passband is varied by controlling the current to an electromagnet which tunes variable frequency resonator elements in the filter across different frequency ranges. Known variable frequency resonator elements include hexagonal ferrite spheres and yttrium-iron-garnet (YIG) spheres.
The advantage of utilizing hexagonal ferrite spheres, instead of YIG spheres, for magnetically tuneable filters in the millimeter wave region is that they have a large internal anisotropy field (H.sub.a), which reduces the applied magnetic field needed to achieve resonance (f.sub.res =2.8 MHz/oersted (H.sub.a +H.sub.applied)). By reducing the required magnetic field, problems associated with electromagnet heating, hysteresis, tuning linearity, and maximum tuning frequency can be reduced.
Iris-coupled, two-sphere, magnetically tuneable millimeter wave bandpass filters fabricated in a waveguide (FIG. 1) utilizing hexagonal ferrite spheres are known. See, for example, Matthaei, G., Young, L., and Jones, E.M.T., "Microwave Filters, Impedance-Matching Networks, and Coupling Structures," Artech House, 1980, pp. 1040-1085; Sweschenikow, J. A., Merinow, E. K., and Pollak, B. P., "Bandfilter aus Hexaferriten im Mikrowellenbereich," Nachrichtechnik Elektronik, 26, 1976, pp. 262-264; and Nicholson, D., "A High Performance Hexagonal Ferrite Tuneable Bandpass Filter for the 40-60 GHz Region," 1985 IEEE MTT-S International Microwave Symposium Digest, pp. 229-232. These filters have been demonstrated to filter across full waveguide bandwidths up through W band, as shown in FIGS. 2, 3, and 4.
The extension of the iris-coupled, two-ferrite-sphere waveguide bandpass filter utilizing one electromagnet to a three- or four-sphere structure having increased off resonance isolation has not, however, been satisfactorily achieved. The aforementioned Sweschenikow, et al., article discloses a three-sphere waveguide bandpass filter utilizing hexagonal ferrite spheres (FIG. 5). The addition of the third sphere did not increase the insertion loss significantly, but only improved the off resonance isolation by approximately 12 dB, while unfortunately spreading the electromagnet pole tips farther apart. In contrast, Fjerstad, R. L., "Some Design Considerations and Realizations of Iris-Coupled YIG-Tuned Filters in the 12-40 GHz Region," IEEE Trans. on Microwave Theory and Techniques, Vol. MMT-8, No. 4, April, 1970, pp. 205-212, discloses a four-sphere waveguide bandpass filter utilizing YIG spheres (FIG. 6). However, this four-YIG-sphere waveguide bandpass filter has a limited bandwidth and poor off resonance isolation at high frequencies. A broadband millimeter wave bandpass filter having high off resonance isolation preferably utilizing hexagonal ferrite spheres is, therefore, needed.